Just tried to spawn five additional copies of my kozilek, butcher of truth, in a hilariously one sided archenemy game. Anyways, it removed mine from the field, didn't spawn any additional ones, reset my graveyard and I got this in a notepad when I got out of the game.

Kozilek is a Legendary Creature. Two or more Legendary permanents with the same name can't exist on the battlefield at the same time.

If you try to put two or more Legendary permanents with the same name on the battlefield at the same time, all of them are sent to the battlefield as a state-based action.

Also, Kozilek shuffles your graveyard into your library when it dies.

Note that the Legend Rule is set to change with the release of Magic 2014; assuming Duels of the Planeswalkers 2014 properly implements the new rule, you would be able to keep one of your Kozileks, and if one of your opponents were also playing Kiora, they would be able to play their own Kozilek without either of them dying.

Excellent, I assumed I would've at least kept my original if a rule like that was in effect. Still, getting the notepad error threw me. All in all, was one of the funniest backfires from any play I've had in dotp, 2012 or 2013.